This invention pertains to articles and systems (collectively xe2x80x9carticlesxe2x80x9d) that comprise a Faraday rotator that does not require a bias magnet.
Faraday rotator materials are a necessary component in non-reciprocal devices such as magnetooptic isolators, circulators and switches. These devices have found extensive application in telecommunications and other fields. The preferred materials for these applications at telecommunications wavelengths are bismuth-doped rare-earth iron gamets. At the principal near infrared telecommunications wavelengths of about 1310 nm and about 1550 nm, these magnetic gamets have a high degree of transparency and reasonably high specific Faraday rotations (Faraday rotation per unit thickness).
Because these bismuth-doped rare-earth iron garnet materials are not congruently melting, nearly uniform crystals can only be grown by flux techniques, typically by liquid phase epitaxy on substrates of non-magnetic garnet. This technology is well reviewed by V. J. Fratello and R. Wolfe (in Magnetic Film Devices, edited by M. H. Francombe and J. D. Adam, Volume 4 of Handbook of Thin Film Devices: Frontiers of Research, Technology and Applications, Academic Press, 2000). As is detailed therein, a close room temperature lattice match of the film to the substrate is required to prevent cracking of the film, the substrate or both. Films are generally grown only on one side of the substrate to allow stress relief by bending. To grow a thick film  greater than 300 xcexcm as is required for telecommunications device applications, the film and substrate lattice parameters should match to within xc2x10.1%xcx9cxc2x10.012 A, preferably within xc2x10.05%xcx9cxc2x10.006 xc3x85. The range of film compositions possible is therefore constrained by the available substrate materials.
Bismuth-doped garnet film compositions have an enhanced Faraday rotation over the pure rare-earth iron garnets. Doping with bismuth strongly effects the electric dipole term in the magnetooptic coefficients through superexchange and spin-orbit interactions (see, for example, P. Hansen and J.-P. Krumme, Thin Solid Films 114, 69 (1984) and H. LeGall, M. Guillot, A. Marchand, Y. Nomi, M. Artinian and J. M. Desvignes, J. Mapn. Soc. Jpn. 11, Supplement S1, 235 (1987)). The total Faraday rotation of the garnet, "THgr"F, may be characterized as the sum of the following:
(1) The iron lattice contribution, which can be determined from the Faraday rotation of yttrium iron garnet (YIG-Y3Fe5O12), "THgr"F(YIG), since yttrium is a non-magnetic ion and does not contribute to the magnetic or magnetooptic properties. This iron lattice contribution is a small positive Faraday rotation.
(2) The rare earth (designated as R) contribution, which can be determined from the difference between the Faraday rotations of YIG and a pure-rare earth iron garnet (RIG-R3Fe5O12), "THgr"F(RIG)-"THgr"F(YIG). Since bismuth substitutes for rare earths in the gamets, this contribution will be diluted by bismuth substitution. (For data on the pure rare-earth iron gamets at telecommunications wavelengths see J. F. Dillon, Jr., S. D. Albiston and V. J. Fratello, J. Magn. Soc. Jpn. 11, Supplement S1, 241 (1987)). These rare earth contributions are typically smaller than the overall iron lattice Faraday rotation and may be positive or negative.
(3) The bismuth contribution, "THgr"F(Bi), which is well characterized by a single line shape (see G. B. Scott and D. E. Lacklison, IEEE Trans. Maqn. 12, 292 (1976)). This contribution is linearly dependent on the bismuth contribution at least up to 2 atoms per formula unit of bismuth (see T. Tamaki, H. Kaneda, T. Watanabe and K. Tsushima, J. Magn. Soc. Jpn. 11, Supplement S1, 391 (1987) and T. Okuda, T. Katayama, K. Satoh, T. Oikawa, H. Yamamoto and N. Koshizuka, Proc. of the Fifth Symposium on Magnetism and Magnetic Materials, ed. by H. L. Huang and P. C. Kuo (World Scientific, Singapore, 1989)). This contribution is large and negative.
These contributions yield the approximate formula:
"THgr"F(BiXR3-XFe5O12)=(X/3)xc3x97["THgr"F(Bi)+"THgr"F(YIG)]+[(1-(X/3)]xc3x97F(RIG)
If a partial substitution of gallium, aluminum or any other diamagnetic ion is made for any of the iron in the garnet, e.g. BiXR3-XFe5-YGaYO12, all these Faraday rotations are reduced by dilution, though not necessarily in a linear manner.
As the bismuth doping is increased, its contribution to the Faraday rotation first cancels the small positive contribution of the rare earth iron garnet (for BiXY3-XFe5O12 this occurs at Xxcx9c0.10-0.15 atoms per formula unit at 1310-1550 nm), then the Faraday rotation increases in magnitude in the negative direction. The upper limit of specific Faraday rotation, "THgr"F/t, results from the maximum allowable bismuth doping in the film. This occurs because of the onset of misfit dislocations in the film, which degrade the optical quality unacceptably (see V. J. Fratello, S. J. Licht, C. D. Brandle, H. M. O""Bryan and F. A. Baiocchi, J. Cryst. Growth 142, 93 (1994)). This maximum bismuth concentration is a complex function of growth conditions and gallium or aluminum substitution but is typically in the range 1.2-1.5 atoms per formula unit.
The range of film compositions that can be grown is constrained by the lattice match to available substrates. Because bismuth has a large ionic size, substrates with higher lattice parameters are required.
Most commonly used and commercially available in large diameters is calcium-magnesium-zirconium substituted gadolinium gallium garnet (CMZ:GGG-{Gd2.68Ca0.32}[Ga1.04Mg0.32Zr0.64](Ga3)O122). The literature value of the lattice parameter of this material is 12.498 xc3x85 (D. Mateika, R. Laurien and Ch. Rusche, J. Cryst. Growth 56, 677 (1982)), but slightly lower values are sometimes quoted as well.
Some thick film crystal growth has been performed on neodymium gallium garnet (NdGG-Nd3Ga5O12) substrates of lattice parameter 12.504 xc3x85. However this material has a significantly worse match of coefficient of thermal expansion to the thick film materials than CMZ:GGG or GSGG. Therefore it is more prone to breakage or catastrophic formation of misfit dislocations (V. J. Fratello, S. J. Licht, C. D. Brandle, H. M. O""Bryan and F. A. Baiocchi, J. Cryst. Growth 142, 93 (1994). For these reasons NdGG is not suitable as a substrate for thick film gamets with high bismuth concentrations.
The next higher lattice parameter commercially available material is gadolinium scandium gallium garnet (GSGG-Gd2.957Sc1.905Ga3.138O12 lattice parameter 12.560 xc3x85 (V. J. Fratello, C. D. Brandle and A. J. Valentino, J. Cryst. Growth 80, 26 (1987)).
Since, as is stated above, good thick film growth requires a room temperature lattice match of xc2x10.012 xc3x85, preferably xc2x10.006 xc3x85, this leaves a considerable lattice parameter range not attainable on commercially available substrates.
To operate most non-reciprocal devices, the magnetic garnet must be maintained in a single domain state. Most magnetic gamets spontaneously demagnetize into multiple domains to minimize their free energy. Device designs have traditionally used a bias magnet to maintain the magnetic garnet in the single domain state required for device operation. However Pulliam et al. (J. Appl. Phys. 53, 2754 (1982)) Identified large stable magnetic domains in garnet films and Brandle et al. (U.S. Pat. Nos. 5,608,570 and 5,801,875) identified the necessary magnetic conditions to maintain such domains over the temperature range of device operation. The specific teachings of the Brandle et al. patents are as follows:
(1) To maintain a saturated magnetic state without a bias magnet, a saturation magnetization 4xcfx80Ms less than 100 G must be maintained over the device operating range, e. g. xe2x88x9240xc2x0 to 85xc2x0 C.
(2) To this end, the material composition must avoid or minimize rare earth ions with a large 1/T temperature dependence of their contribution to the saturation magnetization (e.g. gadolinium (Gd), terbium (Tb) and dysprosium (Dy)).
(3) Because it uniquely reduces the saturation magnetization of the garnet without creating a compensation point, due to its lack of temperature dependence, the concentration of europium (Eu) should be maximized.
(4) Modest concentrations of other magnetic heavy rare earths (e. g. holmium (Ho) through ytterbium (Yb)) can be used to lower the saturation magnetization. Only holmium is effective in this regard for telecommunications applications because erbium has a high absorption in the telecommunications band and the other heavy rare earths (thulium and ytterbium) do not significantly reduce the saturation magnetization. However, no more than modest amounts of holmium are desired because it has a 1/T temperature dependence.
(5) To further adjust the saturation magnetization to the necessary range, doping with ions that preferentially substitute for iron on the tetrahedral site (e. g. gallium (Ga) and/or aluminum (Al)) is required. Because aluminum has a lower site preference for the tetrahedral site, it is less effective in reducing saturation magnetization and has the added disadvantage of reducing the Curie temperature to the detriment of high temperature assembly, storage and operation conditions. If Al is mixed with Ga as a dopant, it also complicates process control of crystal growth. Because of its nonunity distribution coefficient, there is typically a gradient of Al through the film thickness and consequently a variation of film parameters from ideal. Therefore doping with aluminum is contraindicated for making magnedess Faraday rotator materials.
Brandle et al. (U.S. Pat. Nos. 5,608,570 and 5,801,875) identified three compositions, Bi1Eu1Ho1Fe4Ga1O12, Bi0.75Eu1.5Ho0.75Fe4.1Ga0.9O12 and Bi1Eu2Fe4Ga0.5Al0.5O12, all of which approximately match the lattice parameter of CMZ:GGG. Note that all these compositions have europium concentrations C(Eu) greater than or equal to the holmium concentrations C(Ho). Commercially available samples of the preferred composition from Lucent Technologies and Sumitomo Metal Mining have compositions of approximately BiX(Eu0.5Ho0.5)3-XFe5-YGaYO12 with X approximately in the range 1.1 to 1.2 atoms per formula unit and Y in the range 0.9 to 1.0 atoms per formula unit. Once again, these compositions show the concentration of europium approximately equal to the concentration of holmium. In addition, Brandle et al. teaches that only a modest amount of holmium is acceptable, while the europium concentration should be maximized. This clearly suggests that the concentration of europium should be greater than or equal to the concentration of holmium. In addition, all Faraday rotator products currently on the market have C(Eu)xe2x89xa7C(Ho). Therefore, one of skill in the art would understand that the concentration of europium should be greater than or equal to the concentration of holmium.
In the prior art, the degree of bismuth doping is effectively constrained to be X less than 1.2 atoms per formula unit by the following constraints:
(1) The lattice parameter of the garnet film is constrained to match that of CMZ:GGG, effectively 12.486-12.510 xc3x85, preferably 12.492-12.504 xc3x85. There are no hypothesized effective magnetless compositions that lattice match GSGG (gadolinium scandium gallium garnet), which is the next higher lattice parameter (12.560 xc3x85) substrate.
(2) C(Eu)xe2x89xa7C(Ho) places a lower limit on the host rare-earth iron garnet lattice parameter (prior to doping with bismuth and gallium). The lattice parameter of Eu3Fe5O12 is 12.498(3) xc3x85 and that of Ho3Fe5O12 is 12.375(3) xc3x85 (G. P. Espinosa, J. Chem Phys. 37, 2344 (1962)) where the fourth decimal place (in parentheses) is considered approximate. Using Vegard""s law, the lattice parameter of a 1xe2x80x941 solid solution Eu1.5Ho1.5Fe5O12 is approximately 12.437 xc3x85. Solid solutions with C(Eu)xe2x89xa7C(Ho) will have a lattice parameter xe2x89xa712.437 xc3x85. Substitution with Ga slightly decreases the lattice parameter of the host garnet by xe2x88x920.018 xc3x85/(a/fu) so that the nominal lattice parameter of Eu1.5Ho1.5Fe4.05Ga0.95O12 is 12.420 xc3x85.
(3) Bismuth increases the lattice parameter of the film as it is substituted for the combination of rare earths. The measured lattice parameter of a highly defective Bi3Fe5O12 was 12.623 xc3x85 (T. Okuda, T. Katayama, K. Satoh, T. Oikawa, H. Yamamoto and N. Koshizuka, Proc. of the Fifth Symposium on Magnetism and Magnetic Materials, ed. by H. L. Huang and P. C. Kuo (World Scientific, Singapore, 1989)). For the prior art composition BiX(Eu0.5Ho0.5)3-XFe4.95Ga0.95O12 on CMZ:GGG, the range of acceptable lattice matches in constraint 1 has been empirically found to limit X less than 1.2. This can be seen further from the relation                                           C            match                    ⁡                      (            Bi            )                          =                ⁢                  3          xc3x97                                    [                                                LP                  ⁡                                      (                                          CMZ                      :                      GGG                                        )                                                  -                                  LP                  ⁡                                      (                                                                  Eu                        1.5                                            ⁢                                              Ho                        1.5                                            ⁢                                              Fe                        4.05                                            ⁢                                              Ga                        0.95                                            ⁢                                              O                        12                                                              )                                                              ]                        /                                                          ⁢                  [                                    LP              ⁡                              (                                                      Bi                    3                                    ⁢                                      Fe                    5                                    ⁢                                      O                    12                                                  )                                      -                          LP              ⁡                              (                                                      Eu                    1.5                                    ⁢                                      Ho                    1.5                                    ⁢                                      Fe                    4.05                                    ⁢                                      Ga                    0.95                                    ⁢                                      O                    12                                                  )                                              ]                                        =                ⁢                  3          xc3x97                                    [                              12.498                -                12.420                            ]                        /                          [                              12.623                -                12.420                            ]                                                              =                ⁢                  1.15          ⁢                      xe2x80x83                    ⁢          atom          ⁢                      /                    ⁢          formula          ⁢                      xe2x80x83                    ⁢          unit                    
(4) Aluminum must be avoided for the reasons given above. Replacement of all or some of the gallium in the film with aluminum could otherwise be used to reduce the lattice parameter and allow more bismuth doping within constraints 1 and 2.
The largest magnitude specific Faraday rotations commercially available in magnetless Faraday rotator materials (those which do not require a bias magnet) according to the invention of Brandle et al. (U. S. Pat. Nos. 5,608,570 and 5,801,875) are approximately xe2x88x920.09 degrees/micron at 1550 nm. This means that a typical 45 degree Faraday rotator as is required for a magnetooptic isolator will have a thickness of approximately 500 microns for this wavelength (Sumitomo Metal Mining Product Brochure, 2002, Lucent Technologies Product Brochure D S FR.5, July 1998). Additionally a magnetless material not according to the invention of Brandle et al. (containing large amounts of Tb) is manufactured by Mitsubishi Gas Chemical with a specific Faraday rotation of approximately xe2x88x920.096 degrees/micron and a thickness of 470 microns for a 45 degree Faraday rotator at 1550 nm (Mitsubishi Gas Chemical Product Brochure, 2002). However this material has a high saturation magnetization away from room temperature that strongly limits its range of usefulness for magnetless applications.
In contrast, the largest magnitude specific Faraday rotations commercially available in standard Faraday rotator materials (those that require a bias magnet) are xe2x88x920.125 degrees/micron at 1550 nm. This means that a typical 45 degree Faraday rotator as is required for a magnetooptic isolator will have a thickness of 360 microns for this wavelength (Mitsubishi Gas Chemical Product Brochure, 2002). This can be accomplished because more variation in the rare earth composition is possible so that lattice matched standard (high magnetization) Faraday rotator films can be grown on CMZ:GGG with 1.2-1.5 atoms per formula unit of bismuth. Consequently, there is still a need for a Faraday rotator that does not require a bias magnet and yet has a larger specific Faraday rotation than the magnetless Faraday rotators currently available.
A Faraday rotator thick film that does not require a bias magnet and has an improved specific Faraday rotation over the prior art films. Films of nominal composition BiX(EuZHo1-Z)3-XFe5-YGaYO12 are lattice matched and grown on available CMZ:GGG substrates. However, the fraction of europium in the rare earths used is kept less than about 0.45. This allows a higher concentration of Bi to be incorporated into the film, while still keeping the film lattice matched and the saturation magnetization low enough for the film to have a single magnetic domain through its operating range. The higher Bi concentrations give the film a higher specific Faraday rotation. As a result, thinner films can be used in devices that require a rotator with a specific degree of rotation. The thinner films have a shorter path length and improved crystal growth yields over the thicker prior art films.